1. The Field of the Invention
The present invention relates to a system and process for detecting and monitoring surface defects. More particularly, the present invention relates to a process for efficiently detecting and monitoring surface defects on large machined or polished metal surfaces in space shuttle booster motors by analyzing the reflection of multiple light sources directed at regions of the surface.
2. Technical Background
The United States has a long and proud history in the field of space exploration, of which the space shuttle program is an integral part. Space shuttles are lifted into space by reusable rocket booster motors. The outer case of these booster motors is constructed as a stack of interlocked segments, thereby forming a cylindrical container for solid fuel propellant. The case of a typical booster motor used on the space shuttle may include ten or more cylindrical segments in end-to-end relation. After their initial use, the empty booster motors are collected, refurbished, and re-used a dozen or more times.
During flight, these booster motors are subjected to extremely high pressures and temperatures. Because the case of the booster motor is constructed by attaching together several segments, the attachment mechanism must be able to withstand the forces imposed on the case by these extreme temperatures and pressures. Metal-to-metal contact in the attachment areas around the perimeter of the ends of the segments is one important part of the "field joint" which joins each segment with its neighboring segments.
Defects in field joint surfaces may result from a variety of causes, including in-flight stresses, machining and polishing performed during manufacturing and refurbishment, and inherent defects in construction materials. Defects in field joint surface areas are undesirable because they may hamper the desired metal-to-metal contact, and because they may provide initial focal points from which damage due to high pressure or temperature may spread.
Thus, the surfaces of field joints are carefully inspected for defects both during their original manufacture and during each refurbishment. This inspection seeks to detect defects such as bumps, depressions, gouges, burrs, scratches, "fretting", and other unexpected aberrations in the smooth metal surfaces of field joints.
The data produced during defect inspection may be compiled and stored to preserve a record of the defects. Maintaining a record of the defects on field joint surfaces serves dual goals. First, a record of surface defects permits informed decisions on whether a given segment is too defective for further use. Second, an accurate history of particular surface defects in particular segments also provides useful data in evaluating the characteristics of various materials for use in manufacturing or refurbishing container segments.
The most obvious way to inspect a surface is simply to look at it carefully. However, even though such visual inspection may permit the inspection of large areas in relatively short times, it has several drawbacks. Because human inspectors are subject to error and fatigue, inspections vary in thoroughness, accuracy, and speed. Furthermore, visual inspections alone cannot properly track the history of particular defects. Many visual inspections fail to maintain any accurate record of the defects detected. But even if a photograph or sketch is made during the inspection, such records fail to accurately quantify the precise location and extent of defects.
Optical inspection systems based on microscopes, lasers, and other technological aids provide an advancement over simple visual inspection by facilitating the collection and analysis of reflected light. However, most of these previously known approaches to optical inspection are optimized to inspect relatively small surface areas such as semiconductor chip wafers. Such wafers are at least an order of magnitude smaller than container segment field joints, so the use of such previously disclosed systems in connection with field joints or other large surfaces requires impractical amounts of time.
Because they are optimized for small inspection regions, previously disclosed systems often employ only one or two beams of light when illuminating the surface. It is not feasible to simply add more light sources to expand such systems and thereby cover larger regions. For example, many such systems employ illuminating beams which are directed normal to the surface, with optics configured to capture the reflected light for analysis. Adding additional beams to these systems would vastly complicate the optics required to gather and focus the reflected light.
Also, many previously disclosed systems employ lasers to illuminate the surface, because increased coherence in the light directed at the surface facilitates detection of disparities in the reflected light. Adding numerous lasers to increase inspection throughput would prohibitively increase the cost of such a system, and would have significant adverse effects on system size, power and cooling requirements, and other practical factors.
Many previously disclosed inspection systems and processes also fail to maintain any history of particular defects, since the articles being inspected are typically not refurbished or re-used. Accordingly, many previously disclosed systems provide no capability for recording the precise location of each defect. Similarly, many previously disclosed systems are incapable of generating digital maps of the surface which may be compared to track changes in defect location or size.
Therefore, it would be an advancement in the art to provide a system and process which efficiently and economically detects defects in large surface areas such as the field joints of booster motor container segments by gathering and analyzing light reflected off of the surface. It would be particularly beneficial if this advancement in the art did not require the use of lasers as a light source. It would also be advantageous if this advancement did not require a complex optical apparatus to gather the reflected light.
It would be a further advancement in the art to provide a system and process which permits the monitoring of particular defects over time. For instance, it would be particularly beneficial if this advancement in the art included a determination of the surface location of each defect, thereby permitting reexamination of particular defects after the container segment has been re-used. It would also be advantageous if the advancement provided digital maps, thereby permitting computer-assisted management of the defect histories and computer-assisted comparison of records of a given defect which were recorded at different points in time.
Such a system and process is disclosed and claimed herein.